napi_get_cb_info: how to detect instance this and prototype

[ealib]

• Version: 8.1.2
• Platform: Windows 10
• Subsystem: N-API

Maybe a trivial issue, but I found I can't easily distinguish from this retrieved with napi_get_cb_info inside a getter called on an instance of a wrapped class, and the same getter called on that class prototype.

const example = require('example');

let m = new example.MyClass();
console.dir(m); // ok
console.dir(m.prototype); // here getters fail

The getter code fails because napi_get_cb_info returns a thisArg value which can not be used in napi_unwrap (of course).

napi_value MyClass::GetCount(napi_env env, napi_callback_info info) {
    const size_t argc_expected = 0;
    size_t argc = argc_expected;
    napi_value es_this;
    napi_status status = napi_get_cb_info(env, info, &argc, nullptr, &es_this, nullptr);
    assert(napi_ok == status);
    MyClass* native_instance = nullptr;
    status = napi_unwrap(env, es_this, reinterpret_cast<void**>(&native_instance));
    assert(napi_ok == status); // status is napi_invalid_arg when es_this is prototype
    double count = (double) native_instance->count;
    napi_value count_value;
    status = napi_create_number(env, count, &count_value);
    assert(napi_ok == status);
    return count_value;
}

Is checking if (napi_invalid_arg == status) after calling napi_unwrap the only way to detect a getter is called on prototype?

[mscdex]

/cc @nodejs/n-api

[mhdawson]

You may be able to keep the prototype for the wrapped class and check that the this passed in has that prototype. If so then you expect napi_unwrap to work ok.

We don't have an API like is_wrapped, even if we did that might only fix some cases as you might be able to pass in the wrong type of object even if it is one that was wrapped.

[ealib]

@mhdawson since I save a reference to the class constructor, maybe I can use napi_instanceof to find if this is created by that very constructor:

napi_value MyClass_constructor = nullptr;
status = napi_get_reference_value(env, MyClass::es_constructor, & MyClass_constructor);
assert(napi_ok == status);
bool is_instance = false;
status = napi_instanceof(env, es_this, MyClass_constructor, &is_instance);
assert(napi_ok == status);
if (is_instance) {
    // napi_unwrap() ...
} else {
    // otherwise...
}

[digitalinfinity]

@ealib that sounds reasonable- did that workaround work for you or do you need changes in n-api?

[jasongin]

Saving a persistent reference to the class constructor for use with later instanceof checks is a very common pattern for native addons. We should probably document that.

[mhdawson]

@ealib, if that worked for you it would be great to add it to the documentation. If you are interested I could help you submit a PR ? If you don't have time I'll take a look at it.

[ealib]

Sorry for the late reply.

@digitalinfinity yes it works OK so far

@jasongin yes, saving a reference to constructor is a pattern I use for 100% classes because the native module I am working on exposes a few functions to lookup some entities and returned IDs are used to actually get (wrapped) instances of those very entities (another old pattern)

@mhdawson I just updated the module code to use napi_get_new_target/2, but apart that atm I don'thave time for a PR. If not urgent, I can possibly edit the doc by mid October.

[mhdawson]

@ealib, @jasongin created this PR to document. Please review if you have a chance: #16699